Many reusable thermoplastic containers of various sizes and shapes are known in the art which are used for packaging and storing a variety of foods or non-food items. Such containers are useful when storing foods which are to be used up over a period of time because the food is easily resealed in the container with the use of a removable lid or cover.
Reusable, collapsible, adjustable containers are also known which are capable of being collapsed in order to save storage space. For example, a collapsible plastic container is disclosed in U.S. Pat. No. 4,930,644 to Robbins, III where a portion of the container side wall has a reduced wall thickness to permit collapse of the side wall upon the exertion of a compressive force. U.S. Pat. No. 5,226,551 to Robbins, III also relates to reusable, collapsible containers having a peripheral side wall which may be repeatedly moved between collapsed and extended positions.
Such collapsible containers are advantageous because they are reusable and may be partially collapsed as foodstuffs are consumed to save space. In addition, such containers may be compactly stored in a collapsed position, thus maximizing storage space. However, while such collapsible containers provide a convenient storage method, the containers are not necessarily suitable for heating foods in a microwave oven as the containers are usually constructed from a polymer which is not sufficiently heat resistant to maintain the structural integrity of the containers under microwave reheating or cooking conditions. In general, containers made from thermoplastic polymers or copolymers with a softening temperature below 100.degree. C. are not sufficiently resistant to hot foods when heated in a microwave oven, and are subject to softening, which can lead to deformation and leaking.
On the other hand, if the polymer is too rigid, the polymer is not suitable for use in a collapsible container due to the high force required to open the container. Pin holes may also occur in the thinner sections of the container if the polymer is too rigid. For example, U.S. Pat. No. 5,219,628 discloses a multi-layer container suitable for use in microwave cooking comprising a substrate layer of a thermoplastic polymer and an inner protective layer comprising a blend of polymers which contacts the food. However, the polymers comprising the container structure are relatively rigid and are not suitable for use in a collapsible container.
Further, when heated, such as in a microwave oven, many thermoplastic containers are subject to staining by foods, which is undesirable for a reusable container. Another disadvantage of such containers is that they have low optical clarity, making it difficult for a consumer to see the food or liquid stored inside the container.
Accordingly, the need still exists in this art for a thermoplastic container which is flexible enough to be collapsed for compact storage of food or non-food items, rigid enough to withstand microwave heating, and which maintains optical clarity.